Definition from Wiktionary, the free dictionary
- (literally) A male person who has a pleasant, friendly manner.
- (idiomatic, euphemistic) An inoffensive adult male who desires romantic companionship but who is too bland and uninspiring to be attractive.
- 1999, The 'Nice Guy' Syndrome, Self-Help Informational Resources, The Counseling Center for Human Development, The University of South Florida 
- The nice guy, they say, finishes last. But in romantic relationships, the nice guy often isn't even in the running.... The nice guy may have expressed an interest in dating you.... The nice guy is the person who you trust and feel comfortable with, but don't see as "fun" or "challenging" or really all that "interesting," other than as a friend, of course.
- 2002 May 3, Cecelia Goodnow, “What's wrong with being a nice guy? Plenty, according to a local therapist”, Seattle Post-Intelligencer
- "My frustrations were pretty typical Nice Guy relationship issues," he says. "Not feeling appreciated, not feeling sexually desired by my wife. I didn't feel like...I received as much as I gave."
- 2005 September 18, "Dirty D.", Lost Romanticism and the Manwhore, DiaryLand, at uberfrau.diaryland.com
- Nice guys are historically the grail of dating. ... As both Nora and I reminisced and compared how we were treated by the manwhore and by the nice guy, the manwhore always won out.
- 1999, The 'Nice Guy' Syndrome, Self-Help Informational Resources, The Counseling Center for Human Development, The University of South Florida